The present invention relates to a glass funnel for a cathode ray tube used mainly for receiving television broadcast signals and for industrial equipments, and a cathode ray tube using such glass funnel.
A cathode ray tube comprises a glass bulb as a vacuumed envelop which includes basically a glass panel 1 for displaying images and a glass funnel 2 having a neck portion 5 for housing an electron gun 6 as shown in FIG. 7. The major components of the glass funnel 2 are a yoke portion 4 for mounting a deflection coil 7 and a body portion 3 which is contiguous to the yoke portion and extends toward an open end portion for sealing the glass panel 1. Reference numeral 10 designates a sealing portion for sealing the glass panel 1 to the glass funnel 2 with a solder glass or the like and a character A indicates a tube axis connecting the central axis of the neck portion 5 with the center of the glass panel 1.
The inside of the cathode ray tube is maintained under a high vacuum condition because an image is displayed by irradiating electron beams in the glass bulb. Further, it has an asymmetric structure unlike a spherical shape and suffers a differential pressure of 1 atmospheric pressure between the inside and the outside of the glass bulb. Accordingly, there is always a high deformation energy, and therefore, the glass bulb is in an unstable state in structure. When a crack is generated in the glass bulb for the cathode ray tube in such a state, the crack will develop because the high deformation energy tends to be released to thereby cause destruction. Further, in such a condition that a high tensile stress is applied to the an outer surface portion of the glass bulb, a delayed destruction may be resulted due to the function of moisture in the atmosphere, whereby the glass bulb loses reliability.
On the other hand, various kinds of image displaying devices other than the cathode ray tubes have been proposed in recent years. In comparison of the cathode ray tube with the proposed image displaying devices, it has been said that the disadvantages of the cathode ray tube reside in a large depth and a large weight. Therefore, the problems to be urgently solved are to shorten the depth and reduce the weight.
However, in an attempt of shortening the depth of the conventional cathode ray tube, a degree of asymmetry in the structure of the cathode ray tube is increased whereby there arises the problem that a more amount of deformation energy is accumulated in the glass bulb. Further, in an attempt of reducing the weight, an increase of the deformation energy is caused due to reduction in the rigidity of the glass bulb. Since the increase of the deformation energy increases stresses, reduction in safety by the destruction and reduction in reliability by the delayed destruction are accelerated. An attempt to increase the wall thickness of the glass bulb in order to prevent the increase of the stress inevitably results an increase of weight.
It is an object of the present invention to provide a glass funnel for a cathode ray tube which is safe and highly reliable and which can realize the reduction of the dimension in depth and the weight of the glass funnel without increasing stresses in the glass funnel, and to provide a cathode ray tube using the glass funnel.
The inventors of the present invention have made various studies to reduce stresses produced in the glass funnel and have found that when arch-like ridge portions are provided in a body portion of the glass funnel so that both ends of each of the arch-like ridge portions are terminated at or near corner portions of an open end portion of the body portion, a deformation in the body portion can be suppressed, namely, the production of stresses can be suppressed.
In accordance with the present invention, there is provided a glass funnel for a cathode ray tube comprising a funnel-like body portion having a rectangular-shaped open end portion at its one end, a yoke portion formed contiguous to the other end of the body portion and a neck portion connected hermetically to the free end of the yoke portion, wherein the body portion has, on at least its long side, substantially flat arched face portions each of which is substantially perpendicular to an open end face in the rectangular-shaped open end portion and has an arch-like ridge portion whose both ends are extended to portions in the vicinity of corner portions of the open end portion, and the body portion is formed with the arched face portions and a dome-like portion.
The present invention is to suppress a deformation in the body portion by providing arch-like ridge portions in the body portion of the glass funnel to thereby prevent an increase of stresses generated in that portion.
First, description on the relation of the shape of a glass funnel to a stress generated in that portion will be made. In an ordinary cathode ray tube, a neck portion is positioned at the tail in a glass funnel sealed with a glass panel, and a yoke portion is positioned in front of the neck portion wherein a body portion assumes a funnel-like shape so that the body portion gently connects the yoke portion and the glass panel. An open end portion is formed at a front end to provide a sealing portion to the glass panel. The open end portion is in a rectangular shape or a nearly rectangular shape.
It is ideal to bring the shape of the cathode ray tube close to a spherical shape to thereby prevent an increase of the stresses. However, the cathode ray tube has essentially an asymmetric shape because the function of a front face portion to display an image is different from the function of a rear portion for scanning by irradiating electron beams.
Generally, the body portion having a gently curved line in the glass funnel rather has a low rigidity due to the asymmetric shape. In the cathode ray tube having an asymmetric shape, the glass funnel tends to deform by a compressive force in vacuum in a direction of pushing it toward the glass panel, and a tensile stress is generated in the body portion and the sealing portion for sealing the glass panel, which is the weakest in strength. In particular, since a central portion in a long side of the body portion has a relatively low rigidity, a large stress is generated in an open end portion of the body portion. The generated stress reduces safety and reliability on the glass funnel.
In order to reduce such stresses, it is preferred to control a deformation of the body portion on which a force acts so as to push it in the direction of the glass panel. On the other hand, the rigidity of the corner portions at the open end portion of the glass funnel is higher than that of the side portions and a stress generated under a vacuum condition is lower. In order to suppress a possible deformation of the body portion, it is effective to design the body portion of low rigidity to have a shape difficult to deform, or it is effective to support the body portion having a low rigidity with the corner portions having a high rigidity and low stress in vacuum. With such measures, the occurrence of deformation in the body portion can previously prevented or reduced. Further, the concentration of stresses in a portion near the open end portion in the middle of the long side, which is low in rigidity and is apt to deform, can be prevented and therefore, the deformation of this portion can be suppressed.
The present invention is characterized by forming arch-like ridge portions in the body portion so that both ends (end points) of each ridge portion are extended to corner portions or portions near the corner portions in the open end portion of the body portion, whereby a force which may cause the deformation of the body portion is transmitted efficiently to the corner portions in the open end portion. As generally known, an arch-like structure can convert a force applied to an arch into a compressive force acting on a direction along the arch so that the applicable force is transmitted to both ends of the arch. Accordingly, when arch-like ridge portions are provided in the body portion of the glass funnel, an arch-like structure presents a desirable effect of increasing the strength of the body portion.
In the present invention, the arch-like ridge portions are provided by forming the faces which are at sides of the open end portion of the body portion and which extend substantially perpendicular to the open end face, i.e., extend along the direction of the tube axis of the glass funnel. Namely, each of the ridge portions provides the boundary between the funnel-like body portion and a face formed at a side of the open end portion of the body portion. Accordingly, each of the ridge portions presents an arch-like shape wherein both ends of each of the ridge portions reach portions in the vicinity of the corner portions of the open end portion. As a result, at each side of the body portion, an arched face portion having an upper portion which is fringed with the arch-like ridge portion is formed.
It is effective for the arched face portion to have a substantially flat surface structure wherein the major portion of the arched face portion is flat or slightly curved. The body portion of the glass funnel comprises arched face portions in parts and a dome-like curved face (a dome-like portion) in the remainder although the body portion is not gently curved but is in a funnel-like form as a whole. The provision of the arched face portions, i.e., the provision of the surfaces substantially perpendicular to the open end face in the body portion suppresses a deformation in the body portion in the direction of pushing toward the open end portion of the body portion. Accordingly, the arched face portions should be formed in portions on the long side of the open end portion because the portions on the long side are in particular low in strength.
However, a high stress may be produced not only in a long side of the open end portion but also in a short side thereof depending on the shape and the aspect ratio of the glass panel. In such case, arched face portions, i.e., arch-like ridge portions should be formed at the short side in the same manner as described above. In consideration of reducing a burden on the long side, it is preferable to form the arch-like ridge portions on the short side whereby the rigidity of the body portion can be increase as a whole and effects can be promoted.